This invention relates to electrophysiology, and to instrumentation and methods for sensing and analyzing activity of the human heart. More specifically, the present invention relates to a process for sensing and analyzing electrical activity of the human heart utilizing one lead system with a three-channel ECG monitor designed for use with a second lead system that is different from the first.
Over the last several decades, a variety of diagnostic procedures have been developed for sensing and analyzing activity of the human heart. These include electrocardiography, vectorcardiography and polarcardiography, all of which depend upon related instrumentation used to produce records derived from voltages produced by the heart on the surface of the human body.
The records so produced are graphical in character and require interpretation and analysis to relate the resulting information to the heart condition of the patient or other subject. Historically, such records have been produced directly as visible graphic recordings from wired connections extending from the subject to the recording device. With advances in computer technology, it has become possible to produce such records in the form of digitally stored information for later replication or retrieval and analysis. Likewise, with advances in communication technology, remote (wireless) sensing has become possible.
(a) Electrocardiography
The production of a conventional 12-lead electrocardiogram (ECG) involves the placement of 10 lead electrodes (one of which is a ground or reference electrode) at selected points on the surface of a subject""s body. Each electrode acts in combination with one or more other electrodes to detect voltages produced by depolarization and repolarization of individual heart muscle cells. The detected voltages are combined and processed to produce 12 tracings of time varying voltages. The tracings so produced are as follows:
where, in the standard, most widely used system for making short term electrocardiographic recordings of supine subjects, the potentials indicated above, and their associated electrode positions, are:
vL potential of an electrode on the left arm;
vR potential of an electrode on the right arm;
vF potential of an electrode on the left leg;
v1 potential of an electrode on the front chest, right of sternum in the 4th rib interspace;
v2 potential of an electrode on the front chest, left of sternum in the 4th rib interspace;
v4 potential of an electrode at the left mid-clavicular line in the 5th rib interspace;
v3 potential of an electrode midway between the v2 and v4 electrodes;
v6 potential of an electrode at the left mid-axillary line in the 5th rib interspace;
v5 potential of an electrode midway between the v4 and v6 electrodes;
vG (not indicated above) is a ground or reference potential with respect to which potentials vL, vR, vF, and v1 through v6 are measured. Typically, though not necessarily, the ground or reference electrode is positioned on the right leg.
Correct interpretation of an ECG requires a great deal of experience since it involves familiarity with a wide range of patterns in the tracings of the various leads. Any ECG which uses an unconventional system of leads necessarily detracts from the body of experience that has been developed, in the interpretations of conventional ECGs, and may therefore be considered generally undesirable. The tracings generated would be understandable only by a relative few who were familiar with the unconventional system.
Nevertheless, other lead systems have evolved from improvements in instrumentation that have permitted extension of electrocardiography to ambulatory, and even vigorously exercising subjectsxe2x80x94and to recordings made over hours, or even days. For example, in stress testing the electrodes are moved from the arms to the trunk, although the same number of electrodes (10) are used. The tracings I, II, III, aVR, aVL and aVF are altered by this modification.
Although a 12-lead ECG is considered to be a cost effective heart test, it is to be noted that the relatively large number of electrodes required play an important role in determining costsxe2x80x94not only in terms of the direct cost of the electrodes themselves, but also terms of the time required to properly position and fix each electrode to a subject""s body.
(b) Vectorcardiography
The pattern of potential differences on a body surface resulting from electrical activity of the heart can be mathematically approximated by replacing the heart with a dipole equivalent cardiac generator. The magnitude and orientation of this dipole are represented by the heart vector which is continually changing throughout the cycle of the heart beat. The XYZ coordinates of the heart give rise to time varying xyz signals, which may be written out as xyz tracings. Orthogonal leads to give these tracings were developed by Ernest Frank (see An Accurate, Clinically Practical System For Spatial Vectorcardiography, Circulation 13: 737, May 1956). Frank experimentally determined the image surface for one individual, and from this proposed a system using seven electrodes on the body, plus a grounding electrode. The conventional letter designations for such electrodes, and their respective positions were:
E at the front midline;
M at the back midline;
I at the right mid-axillary line;
A at the left mid-axillary line;
C at a 45xc2x0 angle between the front midline and the left mid-axillary line;
F on the left leg;
H on the back of the neck.
The first five electrodes (E, M, I, A and C) were all located at the same transverse levelxe2x80x94approximately at the fourth of the fifth rib interspace. A linear combining network of resistors attached to these electrodes gave suitably scaled x, y and z voltage signals as outputs.
Unfortunately, xyz tracings are not as easy to interpret as 12 lead ECGs. However, Frank intended his system for a different purpose: vectorcardiography.
Vectorcardiography abandons the horizontal time coordinate of the ECG in favor of plots or tracings of the orientation and magnitude of the heart vector on each of three planes: a frontal (xy) plane plotting an x-axis (right arm to left arm) against a y-axis (head to foot); a transverse (xz) plane plotting the x-axis against a z-axis (front to back), and a sagittal plane plotting the y-axis against the z-axis.
Although it has long formed a basis for teaching electrocardiography, vectorcardiography has never become widely used. The technique was demanding and the system of electrode placement was different from that required for the ECG. Extra work was required, and it would still be necessary to record a 12-lead ECG separately with a different placement of electrodes.
An alternative to the Frank lead system that required only four active electrodes (R(right arm), A, F, E), and that used a resistor network based on Frank""s image surface data was proposed in 1958 by G. E. Dower and J. A. Osborne (see A Clinical Comparison of Three VCG Lead Systems Using Resistance-Combining Networks, Am Heart J 55: 523, 1958). However, the xyz signals produced were sometimes different from those of Frank""s lead system, and the RAFE system was not adopted.
(c) Polarcardiography
An alternative representation of the heart vector, known as polarcardiography, has been exploited since the early 1960""s (see G. E. Dower, Polarcardiography, Springfield, III, Thomas, 1971). It has certain inherent advantages in defining abnormalities, and forms the basis of a successful program for automated analysis. Based on xyz signals, polarcardiography employs the Frank lead system. In order to render it competitive with the established 12-lead ECG, the lead vector concept has been employed to derive a resistor network that would linearly transform the xyz signals into analogs of the 12-lead ECG signals (see G. E Dower, A Lead Synthesizer for the Frank Lead System to Stimulate the Standard 12-Lead Electrocardiogram, J. Electrocardiol 1: 101, 1968, G. E. Dower, H. B. Machado, J. A. Osborne, On Deriving the Electrocardiogram From Vectorcardiographic Leads, Clin Cardiol 3: 97, 1980; and G. E. Dower, The ECGD: A Derivation of the ECG from VCG leads (editorial), J. Electrocardiol 17: 189, 1984). The ECG thus derived is commonly referred to as the ECGD. Because the ECGD can be acceptable to an interpreting physician, it is not necessary for the technician to apply the electrodes required for a conventional ECG. Further, associated computer facilities can make vectorcardiograms and other useful displays available from the xyz recordings. Nevertheless, the number of electrodes called for by the Frank lead system are required. In addition, the effort required by the technician recording the xyz signals is about the same as for a conventional ECG.
(d) Conventional Three-Channel ECG Monitor and Lead System
More recently, conventional three-channel ECG monitors have been utilized with a four or five wire cable that transmits input from four or five electrodes placed at selected points on the surface of a subject""s body. Each electrode acts in combination with one or more other electrodes to detect voltages produced by depolarization and repolarization of individual heart muscle cells. The detected voltages are combined and processed to produce three tracings of time varying voltages. Notably, the three-channel ECG monitor does not act simply as a three-channel amplifier but, rather does some signal manipulation to enable staff to visualize the amplified and displayed or recorded ECG in a conventional manner. More particularly, the three channels of a conventional three-channel ECG bedside monitor do the following:
Channel 1, which is connected as the difference between the left arm electrode voltage and the right arm electrode voltage (Vleft armxe2x88x92Vright arm) is simply amplified and presented as conventional ECG lead I.
Channel 2, which is connected as the difference between the left leg electrode voltage and the right arm electrode voltage (Vleft legxe2x88x92Vright arm) is likewise amplified and presented as conventional ECG lead II.
Channel 3, however, is created as the difference between the voltage of an exploring electrode, typically placed on the chest, and the voltage of the xe2x80x9cWilson Central Terminalxe2x80x9d, a construct intended to represent the voltage at the center of the heart. The Wilson Central Terminal is calculated as the average of the voltages of the electrodes attached to the left arm, right arm and left leg. Hence, Vchannel3=Vchestxe2x88x92(Vleft arm+Vright arm+Vleft leg)/3.
If a fifth wire is provided the cable, the electrode is typically attached to the right leg and serves as a ground.
(e) The Dower EASI Lead System
An improved method and apparatus for sensing and analyzing activity of the human heart, and which requires a reduced number of electrodes to produce accurate simulations of conventional twelve-lead electrocardiograms and vectorcardiograms, is described in U.S. Pat. No. 4,850,370 (the contents of which are incorporated herein).
The 4 electrode positions that are fundamental in the use of the Dower lead system consist of electrode position E of the Frank lead system, electrode position A of the Frank lead system, an electrode position S over the upper end of the sternum (manubrium sterni), and electrode position I of the Frank lead system. (Such E, A, S and I electrodes are from time-to-time collectively referred to herein as the xe2x80x9cEASIxe2x80x9d electrodes.) 12-lead electrocardiographic signals and xyz vectorcardiographic signals can be derived by measuring and, with suitable signal processing means, combining and scaling the voltages present between first, second and third selected pairs of the EASI electrodes.
Sometimes it is desirable to be able to utilize one lead system with an ECG monitor designed for use with a second lead system. For example, it may be desirable at times to utilize a conventional three-channel ECG monitor together with the leads from an EASI lead system or, conversely, utilize a monitor and telemetry system designed for use with EASI leads with conventional leads designed for use with a conventional three-channel monitor. In such cases it is desirable to translate data input from one lead system so that it may be processed through the monitor and telemetry system designed for another lead system. The present invention fulfills these needs and provides other related advantages.
The present invention provides a novel apparatus for measuring and processing voltages produced by a human heart as sensed between selected points on the surface of a subject""s body, and a related process for sensing and analyzing electrical activity of the human heart utilizing a first lead system, with an ECG monitor designed for use with a second lead system. Broadly, the process includes the steps of (1) providing a set of electrodes for a first lead system, 2) providing an ECG monitor designed for use with a second lead system that is different from the first lead system, 3) connecting output signals from the first lead system electrodes to channel inputs for the ECG monitor, and 4) transforming the inputted signals to the ECG monitor to output at least one channel signal which is consistent with standard output for the first lead system. The transforming step typically involves a linear transformation of the inputted signals to the ECG monitor to output the at least one channel signal. Such linear transformation includes manipulating differences between the first lead system electrodes and the ECG monitor either before the output signals from the first lead system electrodes are fed to the channel inputs for the ECG monitor, or after the ECG monitor has formed channels.
More particularly, the present invention is well suited for use with a first lead system having at least four electrodes attachable to the subject""s body at predefined locations, and a three-channel ECG monitor which is designed for use with a second lead system that is different from the first lead system. Examples of lead systems to which the process of the present invention may apply include 1) a standard bedside lead system I, 2) a standard EASI lead system, 3) a standard bedside lead system II, and 4) a Dower EASI lead system.
By way of example, the first lead system may comprise an EASI lead system, and the second lead system may comprise a standard bedside lead system I that includes a conventional three-channel ECG monitor. The first electrode (Elec1) is the E electrode, the second electrode (Elec2) is the S electrode, the third electrode (Elec3) is the A electrode, and the fourth electrode (Elec4) is the I electrode of the EASI lead system. In this case, the first electrode (Elec1) and the second electrode (Elec2) are connected to a first channel (Chan1) of the ECG monitor. The third electrode (Elec3) and the second electrode (Elec2) are connected to a second channel (Chan2) of the ECG monitor. Finally, the fourth electrode (Elec4) is connected to the third channel (Chan3) of the ECG monitor and its input is modified by one-third of the summed input from the first three electrodes (Elec1+Elec2+Elec3).
Similarly, and by way of yet another example, the first lead system may comprise a standard bedside system I lead system that provides input for a conventional three-channel ECG monitor, and the second lead system may comprise a standard EASI lead system. Here, the first electrode (Elec1) is a left arm (LA) electrode, the second electrode (Elec2) is a right arm (RA) electrode, the third electrode (Elec3) is a left leg (LL) electrode, and the fourth electrode is an exploring electrode (typically placed on the chest) of the conventional cable for the conventional three-channel ECG monitor. In such case the first electrode (Elec1) and the second electrode (Elec2) are connected to a first channel (Chan1) of the ECG monitor. The third electrode (Elec3) and the second electrode (Elec2) are connected to a second channel (Chan2) of the ECG monitor. The third electrode (Elec3) and the fourth electrode (Elec4) are then connected to the third channel (Chan3) of the ECG monitor to achieve the desired result.
Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.